ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-5-2155-2005 Is there a trend in cirrus cloud cover due to aircraft traffic? Stordal F. 1 2 Myhre G. 1 2 Stordal E. J. G. 2 Rossow W. B. 3 Lee D. S. 4 Arlander D. W. 2 5 Svendby T. 2 Department of Geosciences, University of Oslo, Norway Norwegian Institute for Air Research, Kjeller, Norway NASA/Goddard Institute for Space Studies, New York, New York, USA Department of Environment and Geographical Sciences, Manchester Metropolitan University, UK now at: Bureau of Patents, Oslo, Norway 11 08 2005 5 8 2155 2162 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/5/2155/2005/acp-5-2155-2005.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/5/2155/2005/acp-5-2155-2005.pdf

Trends in cirrus cloud cover have been estimated based on 16 years of data from ISCCP (International Satellite Cloud Climatology Project). The results have been spatially correlated with aircraft density data to determine the changes in cirrus cloud cover due to aircraft traffic. The correlations are only moderate, as many other factors have also contributed to changes in cirrus. Still we regard the results to be indicative of an impact of aircraft on cirrus amount. The main emphasis of our study is on the area covered by the METEOSAT satellite to avoid trends in the ISCCP data resulting from changing satellite viewing geometry. In Europe, which is within the METEOSAT region, we find indications of a trend of about 1-2% cloud cover per decade due to aircraft, in reasonable agreement with previous studies. The positive trend in cirrus in areas of high aircraft traffic contrasts with a general negative trend in cirrus. Extrapolation in time to cover the entire period of aircraft operations and in space to cover the global scale yields a mean estimate of 0.03 Wm<sup>-2</sup> (lower limit 0.01, upper limit 0.08 Wm<sup>-2</sup>) for the radiative forcing due to aircraft induced cirrus. The mean is close to the value given by IPCC (1999) as an upper limit.